The present invention pertains to gradiometer sensors for the measurement of magnetic field gradients.
Portable magnetic field sensors have been used extensively to locate magnetic objects and to magnetically characterize the surface of the Earth. Applications range from the use of a total field magnetometer mounted on an airplane to detect the magnetic field of a submarine, or map the magnetic field of the Earth, to the use of hand-held generators and detectors of low frequency ac magnetic fields to find coins on the beach or cracks in airplane wings.
Using portable magnetic field sensors, one can measure the magnitude of the field, .vertline.B.vertline., with a total field magnetometer, the three vector components of the field, B.sub.x, B.sub.y, and B.sub.z, with a vector magnetometer, or the gradient of the field with respect to direction, G.sub.i,j =.differential.B.sub.i /.differential.j where i,j=x,y,z, with a magnetic field gradiometer. Each measurement choice has implications as to the design of the magnetic field sensor and the ability to convert the raw data measured by the sensor into information about the objects to be detected or the composition of the nearby environment.
Portable sensors inevitably experience small rotations of the sensor as they are moved. In almost all applications, the Earth's field of 50 .mu.T represents a far larger signal than the magnetic field from the object being measured, which typically is less than 1 nT. Total field magnetometers are naturally invariant to rotation, and hence are often used while moving. Portable vector magnetometers have not been used while in motion to measure the self-fields of objects since the rotations of the sensor as it is moved almost always produce far larger changes in the output than the magnetic field of the object of interest. Even the most sensitive systems for measuring and correcting the rotations of a moving platform are not sensitive enough to prevent the signal from a moving vector magnetometer from being contaminated by these rotations. Portable gradiometers are not affected by these rotations because the typical magnitude of the magnetic field gradient of the Earth is usually the same size or smaller than the magnetic field gradients from objects of interest. Hence the rotations of a gradiometer sensor as it is moved are usually not relevant and/or can be easily compensated.
Any magnetic data, once measured, must then be inverted to obtain information about the source. A major limitation of a full field magnetometer is a fundamental mathematical limit in the ability to convert the measured magnetic field magnitude along a typical trajectory of motion (one number vs. position) into information about the location and/or size of the object or objects creating the magnetic field. Both the vector magnetic field (three numbers vs. position) or magnetic field gradient (five numbers vs. position) along typical trajectories, can be used to uniquely locate a small number of magnetic objects and determine their magnetic moment. As discussed above, measuring the vector magnetic field to sufficient accuracy from a moving platform is virtually impossible. On the other hand, it has long been appreciated that a portable magnetic field gradiometer could be used aboard a moving and rotating platform and would provide far superior ability to locate and characterize magnetic objects when compared to a full field magnetometer.
Until recently the most sensitive portable gradiometers have been based on superconducting technology. These sensors usually have a superconducting coil wound in a figure-8 pattern. Such a coil is insensitive to the application of a uniform magnetic field. A gradient in the field, however, creates a persistent current flowing around the coil which can be measured using a superconducting quantum interference device (SQUID). Such systems are extremely sensitive to magnetic field gradients and have been successfully used to locate and identify objects. The difficulty with these systems has been the cost and complexity of making and using superconducting components.
Recently, a new type of gradiometer, the three sensor gradiometer, (TSG), was invented that allows the gradient to be measured by differencing the outputs of two spatially separated sensor magnetometers effectively operating in a constant or zero magnetic field. This TSG, which is described in U.S. Pat. No. 5,122,744 (which is incorporated herein by reference), does not exhibit hysteresis, can be fabricated easily and inexpensively with both high Tc and low Tc superconductive materials and provides a high sensitivity.
In this TSG, a reference magnetometer is rigidly mounted to the sensor magnetometers. The reference magnetometer provides a signal via feedback coils to cancel background magnetic fields (e.g., the earth's relatively large background field) from outputs of the sensor magnetometers. Since the uniform field cancellation is done magnetically, it is not necessary to subtract large voltages to provide a small gradient thus the sensor has excellent linearity and common mode rejection ratio. Two gradient field signals which do not include the large background magnetic field are subtracted from each other electronically and divided by the distance between the field sensors to provide the measured average gradient.
Thus, a high sensitivity may be obtained by a system which is very inexpensive. The gradiometer is particularly advantageous when high Tc superconductive materials are used for fabrication. Either the reference magnetometer and/or one or both of the sensor magnetometers may be a SQUID, but it is not required that it be so. Any vector magnetometer (e.g., a fluxgate magnetometer) may be used to replace any of the reference and/or sensor magnetometers. Higher order gradiometers can be built using a reference magnetometer cube and a plurality of sensor magnetometer cubes.